This invention pertains to the preparation of hydroxyethyl cellulose having enhanced resistance to enzyme catalyzed hydrolysis, and more particularly to the use of an optimum amount of water in the synthesis of a high molar substitution (M.S), greater resistance hydroxyethyl cellulose with low aqueous solution insolubles. This is accomplished by controlling the amount of water available to the cellulose/caustic reaction medium during the ethoxylation reactions wherein the etherification is interrupted at some intermediate M.S. stage and the alkali concentration is lowered before ethoxylation is permitted to proceed to completion.
Water-soluble cellulose ethers are used widely as thickening agents in a variety of water-borne latex paints. However, cellulose ethers are subject to enzymatic hydrolysis when mercurial compounds are removed from the formulations for environmental reasons. The resulting viscosity loss is detrimental to overall coating performance, i.e. an inadequate viscosity at high shear rates results in poor hiding of the coated substrate, an inadequate viscosity at low shear rates results in sagging of the applied paint from a vertical substrate, and loss of viscosity in the stored can leads to pigment sedimentation, i.e. a spoiled paint to the layman.
It is generally theorized that enzymatic instability is a function of the percentage of unsubstituted anhydroglucose units (% uAGU), and in particular to unsubstituted run sequences of three or more of these units in the backbone of a cellulose ether. In turn, the number of unsubstituted anhydroglucose units, at a given degree of substitution is expected to be, in part, related to the extent of chaining, i.e., competitive reactivity between the hydroxyls of added hydroxyethyl groups and the unreacted hydroxyl functions of the cellulose backbone. In agreement with these observations, enzyme resistant hydroxyethyl cellulose has been difficult to produce. This problem has prompted several attempts to prepare enzyme resistant cellulose ethers by utilizing, at least in part, an etherification reagent which reduces or eliminates the competitive chaining reaction. For example, U.S. Pat. No. 3,388,082 discloses the preparation of hydroxypropyl methyl cellulose (HPMC) having increased enzymatic resistance. Unfortunately, this cellulose derivative shows only a moderate increase in stability towards enzymatic hydrolysis and the latex paint industry has found that good color development and compatibility are not possible between hydroxypropyl methyl cellulose and most colorants.
U.S. Pat. Nos. 3,709,876 and 3,769,247 describe a two-step synthesis of cellulose ethers consisting of the methylation of cellulose with methyl chloride, followed by ethoxylation to produce hydroxyethyl methyl cellulose (HEMC). These products are useful as paint thickeners but the formation of inorganic halides in the methylation step and the undesirable hydration characteristics produced by the use of high glyoxal treatment of the final HEMC product, to achieve removal of organic and inorganic byproducts from the high gel-point polymer, are undesirable aspects with respect to the economics of production and to the use of such products in coating formulations.
This undesirability is reflected in U.S. Pat. No. 3,873,518 wherein a description of a process for the production of hydroxyethyl hydroxypropyl methyl cellulose (HEHPMC) is described for the synthesis of an enzymatic resistant derivative, which provides easier manufacturing and purification procedures. Examination of resistance to enzymatic hydrolysis indicates HEHPMC to be inherently less bioresistant than HEMC; it is merely an attempt to resolve the colorant incompatibilities associated with HPMC described in U.S. Pat. No. 3,388,082 and possess only a moderate improvement in enzymatic stability.
In addition, it also is known in the art of synthesizing cellulose ethers that the use of two-stage processes are significant in the production of improved products. Of particular significance to the disclosures of this invention is the use of two-stage processes wherein the etherification reaction is interrupted at some intermediate stage and the product extracted or alkali or other ingredient proportions modified before continuing additional etherification of the product. Such techniques are disclosed in U.S. Pat. No. 2,388,764, 2,667,481, 2,687,411, 3,131,176, 3,131,177 and 3,652,539. They are described as useful in the production of cellulose ethers through general improvement in such properties as solution clarity, minimization of insoluble gels and as improved procedures for extraction of cellulose ethers during the intermediate stages of production.